JP3278982B (Patent reference 1) and JP2008-208788A (Patent reference 2) disclose known piston pumps which pump fluid (brake fluid) to be sucked into a pump chamber by itself and are adopted to a brake hydraulic pressure control apparatus for a vehicle which has an electronic control function such as an ESC (electronic stability control).
The known piston pumps disclosed in Patent reference 1 and Patent reference 2 include an annular groove provided at an outer periphery of a piston whose one end (a front end) faces the pump chamber, a feed ring mounted to the annular groove, a feed chamber formed between the feed ring and a housing which houses the piston, and a valve portion formed between the piston and the feed ring for opening and closing an inlet side passage of the feed chamber by contacting the feed ring to a lateral surface of the annular groove (groove surface). The valve portion is closed during an intake stroke by the piston, and a volume of the feed chamber is reduced so that the fluid flowed in the feed chamber is pumped into the pump chamber.
Here, an end portion of the piston which faces the pump chamber is defined as a front end of the piston and an opposite end portion is defined as a rear end of the piston. Further, an axial direction and a radial direction here correspond to an axial direction and a radial direction of the piston, respectively.
The piston pump disclosed in Patent reference 1 includes a passage formed between a bottom of the annular groove provided on the piston and an inner peripheral surface of the feed ring. According to the disclosure, a fluid sucked through the valve portion flows into the passage via an inlet port during a compression stroke of the piston, and thereafter, the fluid is introduced to the feed chamber via a passage formed within the piston.
Further, according to the piston pump disclosed in Patent reference 2, a slit, which allows a passage provided between the annular groove and an outer periphery of the piston to communicate with the feed chamber, is formed on the feed ring. According to the disclosure, by providing a path for introducing the fluid to the feed chamber via the slit in addition to a path provided inside the piston for introducing the fluid to the feed chamber, an intake path for the feed chamber is expanded.
As explained above, the piston pumps disclosed in Patent reference 1 and Patent reference 2, or the like, include the passage for flowing the fluid between the bottom of the annular groove formed on the piston and the inner peripheral surface of the feed ring. According to the foregoing construction, a radial gap corresponding to the dimension of the passage is generated between the piston and the feed ring. Thus, a radial displacement (positional displacement) of the feed ring from the center of the piston is allowed, which makes it difficult to assemble the piston and the feed ring to the housing because of the radial displacement of the feed ring.
The feed ring is mounted to the annular groove of the outer periphery of the piston in advance, and both the feed ring and the piston are simultaneously inserted into an assembling hole provided at the housing to be assembled to the housing. In those circumstances, in a case where the feed ring is configured to be displaced in a radial direction from the center of the piston, a portion of the feed ring may be caught at an opening of the assembling hole or at a stepped portion provided inside the assembling hole when inserting the piston into the housing. Thus, the piston may not to be introduced into the assembling hole in a right manner, thereby undermining an assembling performance.
Further, in a case where the piston is forced into the housing, the feed ring may be damaged by the piston and the housing therebetween.
A need thus exists for a piston pump which is not susceptible to the drawback mentioned above (A need thus exists for a piston pump which includes a feed ring outfitted to an outer periphery of a piston and pumps fluid of a feed chamber into a pump chamber during an intake stroke by the piston, and which enhances an assembility of the piston and the feed ring to the housing by enabling a retention of the feed ring at the center of the piston).